The present invention relates to electrically operated devices and appliances and, more particularly, to a power-operated device which can operate in a cordless mode from a self-contained power source or in a corded mode from a conventional AC or DC power source.
Various designs have been developed for electrically operated devices and appliances that receive their electrical energy from a rechargeable battery. In some systems, the rechargable battery is contained within the device housing as non-removable component, and battery charging is effected by mechanically and electrically connecting the device to a self-standing charger that provides charge current through an electrical connection interface. In addition to designs which provide a charger as a separate self-standing unit, the charger can take the form of a unit that is plugged into a conventional wall outlet and connected to the device to be charged by a plug-type connector. In other systems, the rechargeable battery is contained within a removable battery pack that is normally contained, for example, in the handle portion of the device and removed for connection to the charger unit. Lastly, some designs provide for the rechargable battery and an internal battery charger as non-removable components within the device housing. The device can be operated in a corded mode from a conventional wall outlet, in a cordless mode during which the device is powered by its battery, and in charge mode during which the internal charger functions to charge the battery. The devices which employ internal chargers have typically been low-power devices, such as hand-held calculators, portable computers, and radio transceivers.
Advances in battery technologies and systems have led to the development of physically compact and electrically efficient batteries that store a relatively large amount of electrical energy. These higher energy batteries have permitted the development of higher power cordless devices that operate at power levels above 50 watts, including power levels that extend to 500 watts or so. These higher power devices include, for example, portable hand-held power tools and appliances such as drills, screwdrivers, screwdriver-drills, hammer-drills, jig saws, circular saws, shears, hedge trimmers, and various household products and appliances including vacuum cleaners.
Certain design criteria are presented when designing an internal battery charger for charging the battery associated with cordless power devices. In low power systems, a corresponding low-current charger can be readily provided which presents a low-current to the battery with recharging effected over many hours. Since the powered devices typically operate in the 3-48 volt DC range, a step-down transformer is typically used to step the 120 or 220/240 VAC input current to a lower level for rectification to a DC value suitable for battery recharging. In the low current chargers, relatively small and inexpensive components are available to effect the voltage conversion to allow the incorporation of the charger circuitry into the cordless device as an internal charger. As the quantity of electrical energy stored in the battery increases, the corresponding size and cost of the electrical components for the charger, especially the step-down transformer, increases proportionately. Accordingly, the corresponding increase in the physical size of the charger components for a reasonable charge current tends to define a practical limit for the incorporation of the charger in the cordless device as an internal charger. Thus the incorporation of an internal charger into a low-power device, such as a hand-held calculator, represents a reasonable design choice because of the small physical size and low cost of the charger components. However, the incorporation of an internal charger in a higher power devices, such as high power motorized devices including power tools and household appliances, requires the need to incorporate high current devices, particularly the step-down transformer, into the cordless device. As can be appreciated, the incorporation of such devices adds to the weight of the device and represents a design constraint in providing a compact device. Thus in high-power dual-mode corded/cordless devices, a practical design constraint exists with regard to the incorporation of an internal charger which does not compromise the utility of the device.